The invention is particularly applicable to telecine. In essence, flying spot telecine operates by imaging a cathode ray tube raster onto a film, collecting the light passing through the film and converting the collected light into a television signal representative of the images on the film. One problem with such telecines is afterglow, which is the persistence of light from the cathode ray tube phosphor after the spot has moved. When the film changes from clear to dense, the video output should immediately change to a minimum. However, afterglow effects cause a decay from peak to minimum signals. This decay shows itself as a decaying white streak to the right of any white information on a black picture. In existing machines, the afterglow characteristic of the phosphor used decays to 10% in about 150 nanoseconds but continues to emit smaller proportions of light which remain significant for about 50 microseconds.
For a infinitely small clear spot on an otherwise dark film, picture streaking will follow the phosphor decay curve. However, if a larger spot of clear film is considered the streaking will correspond to the integral of the phosphor decay curves over the exposed scan time. To overcome this unwanted streaking it has been common practice to correct for afterglow using a series of individually adjustable differentiating circuits in an analogue corrector circuit.
Previously, the afterglow correcting circuitry has been the only remaining analogue component of a telecine machine. It would be desirable to provide a digital afterglow corrector that was compatible with the remainder of the telecine. However, although the existing analogue correctors could be simulated using equivalent digital circuits, these would be very complex with little direct benefit.
Flare is a phenomenon which is present in all flying spot scanners. Flare may be divided into three broad categories; high frequency flare, low frequency flare and flare ringing at intermediate frequencies. Flare rings are known to be caused by changes of refractive index at the glass/air interface at the face plate. This causes light to be reflected back to the phosphor screen at different positions producing a ring around the spot. Although it is not possible to say exactly what causes high and low frequency flare it is clear that both types are produced from a number of different sources which interact with one another, for example general background light and local scattering at the glass-to-air interface. This latter type varies with the image position and is much greater at the edge of an image and tends to smear the image. Multiple reflections tend to produce higher frequency flare.
Although flare can be reduced to an extent by using high quality faceplates and by increasing the thickness of the faceplate as described, for example, in our European patent application EP-A-0266154, flare remains a problem.